would be great to see Stadia being available for more countries, but at the moment Stadia is always just replying
"Our goal is to make Stadia available to as many people as possible. We'll be sharing more details on additional countries in the near future."
Therefore the only thing you can do at the moment is to leave feedback via stadia.google.com or in the Stadia App to make them aware that you would like to get Stadia available in South Africa.
You can check this page from time to time to see if South Africa will be added in the future, or follow Stadia news here in the community or their social media channels. I'm sure when more countries are announced, we'll here it everywhere.